The present invention relates to a phase change memory cell, and more specifically, to a chemical mechanical polishing (CMP) stop layer for fully amorphous phase change memory pore cell. By fully amorphous phase change memory pore cell it is meant that the phase change material, which is located within the pore, is completely converted to the amorphous phase leaving no partially crystalline phase change material within the volume of the pore during the reset operation (where the reset operation is the operation which converts the state of the memory cell to the high resistance state).
High temperature data retention is a desirable characteristic for phase change memory. Retention is limited by unintended recrystallization of the amorphized phase change material. Crystallization can occur at an amorphous-crystallize boundary. The absence of an amorphous-crystalline boundary within the phase change material eliminates this cause for data loss. Hence, a phase change memory cell which converts all of the phase change material into the amorphous state during the reset operation will have better data retention characteristics. In phase change memory, data is stored by causing transitions in the phase change material between amorphous and crystalline states using current. Current heats the material causing transitions between the two states. The change from the amorphous state to the crystalline state is a low current operation in comparison to the change from the crystalline state to the amorphous state (which is referred to as a reset current). It is desirable to minimize the reset current.
FIGS. 1A through 1I illustrate a method for fabricating a conventional phase change memory pore cell 1. Specifically, FIGS. 1A through 1I illustrate a typical keyhole transfer method. In FIG. 1A, a bottom electrode layer 10 and a memory cell layer 12 on top of the bottom electrode layer 10 are provided. The bottom electrode layer 10 includes a dielectric fill layer 13 and a bottom electrode 14 typically made of tungsten or titanium nitride, within the dielectric fill layer 13. A first dielectric layer 15 is formed on the bottom electrode layer 10 and an isolation layer 16 is formed on the first dielectric layer 15, and a second dielectric layer 17 is formed on the isolation layer 16. A photo resist layer 18 is formed over the second dielectric layer 17. A via 20 is formed to extend to the first dielectric layer 15. In FIG. 1B, the photo resist layer 18 is removed and the isolation layer 16 is recessed, creating overhang portions 17a and 17b of the second dielectric layer 17. In FIG. 1C, a conformal film 22 is deposited within the via 20 and pinched to form a void (i.e., a keyhole structure 24) in a lower region of the via 20. In FIG. 1D, the conformal film 22 is recessed and the keyhole structure 24 is transferred down into the first dielectric layer 15 to form a pore 26. In FIG. 1E, the isolation layer 16, the second dielectric layer 17 and the conformal film 22 are removed thereby exposing the pore 26 formed within the first dielectric layer 15. In FIG. 1F, phase change material 28 is deposited over the first dielectric layer 15 and filled in the pore 26. Next, in FIG. 1G, a planarizing process is performed to remove the phase change material 28 formed outside of the pore 26. Next, in FIG. 1H, a top electrode layer 30 is then formed over the first dielectric layer 15. In FIG. 1I, the top electrode layer 30 is then etched to form a top electrode 31 which is in electrical communication with the pore 26 and the bottom electrode 14.
There are several problems associated with the fabrication method shown in FIGS. 1A through 1I. For example, after the pore 26 is etched and the isolation layer 16, the second dielectric layer 17, and the conformal film 22 are removed, in order to ensure sufficient electrical conduction between the phase change material 28 and the bottom electrode 14, a sputtering process is utilized prior to the phase change material 28 deposition. As shown in FIG. 1F, the sputtering process increases a taper angle 26a and 26b and rounded top corners 26c of the pore 26. The more rounded the pore 26, the higher the reset current required in order to make the phase change memory pore cell 1 fully amorphous. If the pore 26 is too rounded then the cell 1 may fail to become fully amorphous.